1 3
Topics in Current Chemistry (2018) 376:43
electrooxidative glycerol cascade (resulting in mesoxalic acid), while carboxylated multiwalled carbon nanotubes (MWCNT-COOH) were capable of electrooxidatively cleaving CO 2 from mesoxalate, and an oxalate decarboxylase enzyme
was employed to hydrolytically cleave oxalate into CO 2 and formate (which was
subsequently oxidized by TEMPO-LPEI) [29, 30]. The net result was a collection
of 14 electrons per molecule of glycerol [30, 31] (Fig. 4).
4 Applications of Electroenzymatic Cascades
While the use of hybrid TEMPO/enzyme catalytic cascades has subsequently
expanded to a variety of additional short-chain polyols, other hybrid catalytic cascades have employed combinations of immobilized redox enzymes with a variety
of nanoparticles to enhance electrocatalytic rates and enable energy extraction
from electrochemically inaccessible biofuels. Wang et al. used N-hydroxysuccinimide (NHS-) activated pyrene butyric acid to immobilize glucoamylase onto
Au nanoparticle-decorated carbon nanofibers to hydrolyze starch to glucose and
subsequently oxidize glucose to gluconolactone [32]. This enabled an array of
multiple glucoamylase enzymes (which exhibits relatively sluggish kinetics) to
be immobilized around relatively few Au nanoparticles on the carbon electrode
surface.
In addition to energy conversion, the use of electrocatalytic cascades has been
used extensively in the development of biosensors. Enzymatic and hybrid cascades have been employed to generate an electrochemical signal transduction
pathway from electrochemically silent analytes. In an interesting demonstration
of this approach, an electrometabolic cascade described recently by Kopiec et al.
employs a bienzymatic system for the detection of phosphate [33]. In this biosensor, purine nucleoside phosphorylase (PNP) utilizes dissolved inorganic phosphate as an analyte to phosphorylate a sacrificial inosine molecule. Upon phosphorylation, inosine is cleaved into ribose-1-phosphate and hypoxanthine, which
is subsequently oxidized twice by xanthine oxidase (XOx) to generate four electrons while producing uric acid. The generated uric acid is then directly oxidized
at the electrode to generate a total of six electrons per molecule of inorganic
phosphate (Fig. 5).
It should be noted that there are numerous reports of electrochemical biosensors employing a combination of glucose oxidase (which oxidizes glucose to gluconolactone and produces H 2 O 2 ) with horseradish peroxidase (HRP) to reduce
H 2 O 2 , where HRP is interfaced at an electrode via an electrochemical mediator.
However, because the parent substrate (glucose) does not react further, these
systems are not strictly considered cascades based on the criteria used herein.
Therefore, the specifics of this literature will not be discussed here further. While
the majority of examples to this point have focused on oxidative electrometabolic pathways, there has been a growing interest in hybrid catalytic motifs in the
design of reductive electrometabolic pathways.
Reprinted from the journal
157
Topics in Current Chemistry (2018) 376:43
electrooxidative glycerol cascade (resulting in mesoxalic acid), while carboxylated multiwalled carbon nanotubes (MWCNT-COOH) were capable of electrooxidatively cleaving CO 2 from mesoxalate, and an oxalate decarboxylase enzyme
was employed to hydrolytically cleave oxalate into CO 2 and formate (which was
subsequently oxidized by TEMPO-LPEI) [29, 30]. The net result was a collection
of 14 electrons per molecule of glycerol [30, 31] (Fig. 4).
4 Applications of Electroenzymatic Cascades
While the use of hybrid TEMPO/enzyme catalytic cascades has subsequently
expanded to a variety of additional short-chain polyols, other hybrid catalytic cascades have employed combinations of immobilized redox enzymes with a variety
of nanoparticles to enhance electrocatalytic rates and enable energy extraction
from electrochemically inaccessible biofuels. Wang et al. used N-hydroxysuccinimide (NHS-) activated pyrene butyric acid to immobilize glucoamylase onto
Au nanoparticle-decorated carbon nanofibers to hydrolyze starch to glucose and
subsequently oxidize glucose to gluconolactone [32]. This enabled an array of
multiple glucoamylase enzymes (which exhibits relatively sluggish kinetics) to
be immobilized around relatively few Au nanoparticles on the carbon electrode
surface.
In addition to energy conversion, the use of electrocatalytic cascades has been
used extensively in the development of biosensors. Enzymatic and hybrid cascades have been employed to generate an electrochemical signal transduction
pathway from electrochemically silent analytes. In an interesting demonstration
of this approach, an electrometabolic cascade described recently by Kopiec et al.
employs a bienzymatic system for the detection of phosphate [33]. In this biosensor, purine nucleoside phosphorylase (PNP) utilizes dissolved inorganic phosphate as an analyte to phosphorylate a sacrificial inosine molecule. Upon phosphorylation, inosine is cleaved into ribose-1-phosphate and hypoxanthine, which
is subsequently oxidized twice by xanthine oxidase (XOx) to generate four electrons while producing uric acid. The generated uric acid is then directly oxidized
at the electrode to generate a total of six electrons per molecule of inorganic
phosphate (Fig. 5).
It should be noted that there are numerous reports of electrochemical biosensors employing a combination of glucose oxidase (which oxidizes glucose to gluconolactone and produces H 2 O 2 ) with horseradish peroxidase (HRP) to reduce
H 2 O 2 , where HRP is interfaced at an electrode via an electrochemical mediator.
However, because the parent substrate (glucose) does not react further, these
systems are not strictly considered cascades based on the criteria used herein.
Therefore, the specifics of this literature will not be discussed here further. While
the majority of examples to this point have focused on oxidative electrometabolic pathways, there has been a growing interest in hybrid catalytic motifs in the
design of reductive electrometabolic pathways.
Reprinted from the journal
157